The invention relates to electric machines in general and more particularly to an arrangement for deep cooling field winding, in particular a superconducting field winding, in the rotor of an electric machine, particularly of a turbogenerator.
Such arrangements with a corotating mixing chamber which contains a coolant mixture of vaporous and liquid components supplied from the outside through a feed line and having a first discharge line for the discharge of a first liquid coolant stream to cool the field winding and, near the rotor shaft, another coolant discharge line running between areas of different temperature, the second discharge line being discharged in the form of a centrifugal pump to pump another gaseous coolant flow out of the mixing chamber and containing at least one colder line section which runs essentially radial relative to the rotor shaft, leading away from the shaft, and warmer line sections leading back to the vicinity of the shaft is described in "Cryogenics", July 1977, pages 429 to 433.
This cooling arrangement is based on a cooling device such as described in the dissertation by A. Bejan entitled: "Improved Thermal Design of the Crygoneic Cooling System for Superconducting Synchronous Generator", Ph.D. thesis, Massachusetts Institute of Technology (USA), December 1974, pages 145 to 167. According to this cooling device as illustrated by FIG. 1A, a coolant taken from an external coolant supply unit is fed through a central feed line 103 to a corotating, centrally disposed mixing chamber 3. This mixing chamber contains a two phase mixture of a liquid coolant A and gaseous coolant B. In the operating state, when rotating, the phases of this two phase mixture are separated by the centrifugal forces acting upon them, and the coolant vapor B accumulates in mixing chamber areas near the shaft axis 5; the liquid coolant A accumulates in mixing chamber areas away from the shaft. A liquid coolant stream flows from the mixing chamber towards the field winding 105 through radially disposed feed lines 107. The coolant then flows through the field winding, for instance, in a direction parallel to the rotor shaft. Since it is heated due to dissipation occuring there or through heat transfer from the outside, its density decreases correspondingly. This causes a hydrostatic pressure difference between the radial feed and return lines, resulting a connective flow and causing the coolant to flow back into the mixing chamber through another, radially directed return line 109. The heat absorbed leads to a temperature rise and to a partial evaporation of the coolant. The pumping action required for the coolant to flow through the field winding is thus brought about by this thermo-siphon effect based on density differences.
It is expedient to keep the coolant in the mixing chamber, required to cool the field winding, at an underpressure so that it will boil at a relatively low temperature. According to the "Cryogenics" literature reference cited, the coolant discharge line may be designed as a pumping device, utilizing the rotation and the heating of the coolant to produce the desired underpressure in the mixing chamber. In this pumping device also called a centrifugal pump, a cool coolant gas taken from the mixing chamber near the rotor shaft is conducted in one or more cold radial tubes 7 to a larger radius, thereby compressing it under the effect of centrifugal force. Then it is heated in a heating section 9 extending essentially parallel to the rotor shaft and subsequently returned through a warm radial tube 12 to the vicinity of the shaft axis, where it is conducted out of the rotating part of the machine through a rotating helium coupling 111 and fed to a refrigeration machine. The coolant is expanded again on its way to the shaft from the heating section remote from the shaft. On account of the different coolant density, the pressure difference between the points closest to and most remote from the shaft is greater on the cold side than on the warm side. If approximately atmospheric pressure prevails at the discharge point, the desired underpressure is then obtained in the mixing chamber with this self-pumping system of the centrifugal pump.
It has now been found in experiments that while an underpressure can be produced briefly with such a centrifugal pump, practically stable underpressure conditions are not obtainable in this manner over longer periods of time. For, as soon as or shortly before the calculated final pressure is reached, the pressure in the mixing chamber generally again suddenly rises to atmospheric pressure (see "Proc. of the VIIth International Cryogenics Engineering Conference" (ICEC 7), London 1978, pages 373-377, for instance). The reason for the occurrence of these instabilities and measures for the stabilization of the underpressure are not known to date, however.
Therefore, it is an object of the present invention to improve the arrangement for cooling a superconducting field winding of the type described at the outset so that a stable underpressure can be maintained in the mixing chamber with that part of the second coolant discharge line which is designed as a centrifugal pump.